It was hypothesized that immunoglobulin (Ig) in normal human serum attach to the surface of senescent RBC and that the quantity of attached Ig increased with increasing age of RBC until a critical level is reached, at that time macrophages no longer can recognize these RBD as "self" cells. In this investigation, an attempt was made to adhere as closely as possible to in situ conditions using short term culture techniques. It was proven both by indirect and direct methods that macrophages can differentiate "adult self" on the basis of selective IgG attachment to the surface of senescent cells.